1. Field of the Invention
The present invention relates to an apparatus and a method for controlling shifting operations of an automatic transmission for an automotive vehicle, and more particularly to improvements in such apparatus and method, for reducing shifting shocks of the transmission when the shift lever is moved from its neutral position to one of its forward and reverse drive positions.
2. Discussion of the Prior Art
U.S. Pat. No. 3,738,199 discloses an electronically controlled automatic transmission, in which an operation of the transmission shift lever from its neutral position to one of its forward drive positions causes the automatic transmission to be shifted to a transient position, which is one of the speed positions of the transmission other than the first-speed or lowest-gear position, in order to reduce the shifting shock. This manner of controlling the automatic transmission is referred to as "anti-squat shifting control", since the squatting of the vehicle upon shifting of the shift lever from the neutral position to a forward drive position can be prevented. More specifically, when the shift lever is moved from its neutral position, appropriate frictional coupling devices of the automatic transmission are commanded to be engaged to establish the transient position (other than the first-speed position). The transmission is held in the transient position for a predetermined suitable time (e.g., 0.8 second) after the operation of the shift lever from the neutral position. Then, the transmission is commanded to be shifted to the first-speed position.
For example, a transmission shift lever has three forward drive positions "D", "S" and "L", while a transmission has a total of four speed or gear positions, "1st-speed", "2nd-speed", "3rd-speed" and "OD" (overdrive position), as indicated in FIG. 3. If the shift lever is shifted from the neutral position "N" to the DRIVE position "D", by way of example, the transmission is shifted to the 1st-speed position by operating the clutches C0 and C1 to the engaged position, if the anti-squat shifting control is not effected. If the transmission is operated in the anti-squat shifting control mode, however, the transmission is first shifted to its 2nd-speed position, with the clutches C0 and C1 and brake B2 brought to the engaged position, for instance. Thereafter, the brake B2 is disengaged or released, to shift the transmission to the 1st-speed position.
According to the anti-squat shifting control as indicated above, the shifting shock of the transmission is reduced by an amount corresponding to a difference in the gear ratio between the 1st-speed and 2nd-speed positions. Further, the degree of the squatting phenomenon of the vehicle (i.e., lowering of the stern or tail of a vehicle) upon starting of the vehicle can be significantly reduced.
Laid-open Publication No. 61-116160 of unexamined Japanese patent application discloses a technique in which the anti-squat shifting control is effected only when the running speed of the vehicle is zero and when the speed of the engine exceeds a predetermined upper limit. This arrangement is derived from the recognition that the transmission undergoes a particularly large shifting shock when the shifting of the shift lever from its neutral position to the DRIVE position occurs at a relatively high speed of the engine, i.e., when the vehicle is started (i.e., shifted into a drive gear) with the engine speed at a relatively high level. In other words, the arrangement is based on the concept that the vehicle should be started fast when the shifting of the shift lever from the neutral position to the DRIVE position occurs while the engine speed is relatively low.
In the conventional anti-squat shifting control arrangements discussed above, the anti-squat shifting of the transmission is accomplished unconditionally in response to any shifting operation of the shift lever from the neutral position to one of the drive positions, or alternatively the determination as to whether the anti-squat shifting is effected is based on only the vehicle speed and the engine speed at the time of the shifting of the shift lever from the neutral position. In the latter case according to the laid-open Publication No. 61-116160, the anti-squat shifting is not effected if the vehicle and engine speeds upon operation of the shift lever from its neutral position are not satisfied. Namely, the transmission is not controlled in the anti-squat shifting mode, even if the above-indicated conditions are satisfied a short time after the operation of the shift lever from the neutral position.
Recently, there is a growing demand for "full-time" 4-wheel drive vehicles which are always driven by four drive wheels. These 4-wheel drive vehicles wherein the engine power is distributed to the four wheels experience a reduced power loss of the engine, as compared with the ordinary 2-wheel drive vehicles, even when the vehicles are started with the transmission placed in the low-gear position and the engine running at a high speed. In the 2-wheel drive vehicles, the wheel tires may slip on the road surface when the drive torque exceeds the total friction force between the two drive wheel tires and the road surface. In the 4-wheel drive vehicles, however, the four drive wheels are less likely to slip on the road surface because of the distribution of the drive torque to the four wheels, whereby the power transmission system including the automatic transmission should have increased strength sufficient to withstand the relatively large load, particularly at the time of abrupt starting of the vehicles at relatively high speeds of the engine. Therefore, the 4-wheel drive vehicles require accurate anti-squat shifting control of the automatic transmission. Accordingly, the monitoring of the conditions in which the anti-squat shifting of the transmission is effected should be more suitably carried out, in the 4-wheel drive vehicles.
The engine speed as one of the conditions to effect the anti-squat shifting control of the transmission may be replaced by the currently required output of the engine, which is, for example, a currently detected amount of operation of the accelerator pedal of the vehicle. If the accelerator pedal is monitored upon shifting of the shift lever from the neutral position, or for a very short time after the shifting of the lever, the determination as to whether the operation amount of the accelerator pedal exceeds a predetermined limit or not is unstable and unreliable, since the accelerator pedal may be further depressed after the monitoring time is over. In this situation, the anti-squat control of the automatic transmission may be not properly accomplished so as to minimize the shifting shock of the transmission and the squatting of the vehicle.